1 Chronicles 29:9
9 The people were filled with joy, because they had given willingly — wholeheartedly they had given willingly to Adonai; and David the king too was completely filled with joy,
This is the secret to giving, do it willingly. In this case the joy the people felt contributing to the construction of the temple caused King David to bless the nation. By giving willingly Adonai fills our lives with joy allowing us to witness to others.